1. Field of the Invention
The present invention relates to a piezoelectric pump preferably for use in transporting fluid by applying a voltage to a piezoelectric element in order to bend a diaphragm.
2. Description of the Related Art
Piezoelectric micro pumps are used, for example, as coolant supply pumps in small electronic appliances, such as notebook PCs or as fuel supply pumps in fuel cells. Piezoelectric micro pumps utilize diaphragms that bend when a voltage is applied to piezoelectric elements. Piezoelectric micro pumps have advantages in that they have simple structures, low-profile shapes, and low power-consumption properties.
Conventionally, a diaphragm unit in a piezoelectric micro pump, which includes a diaphragm on which a piezoelectric element is bonded, typically has a unimorph structure, in which a single-layer piezoelectric element is bonded to the diaphragm, or a bimorph structure, in which a multilayer piezoelectric element is bonded to the diaphragm. In particular, to drive the diaphragm unit at a relatively low frequency in a hydraulic pump, for example, the bimorph structure is more efficient than the unimorph structure. To drive the diaphragm unit at a high frequency in a pneumatic pump, for example, the unimorph structure is advantageous.
In the unimorph structure, a piezoelectric element that expands and contracts in directions parallel to the surface thereof is bonded to a diaphragm that does not expand and contract. The unimorph structure usually has a neutral plane with respect to bending in the piezoelectric element. The term “neutral plane” refers to a plane in which a compressive stress and a tensile stress in the diaphragm unit are zero, when the entire diaphragm unit bends. When a neutral plane exists in the piezoelectric element, a portion of the piezoelectric element moves so as to resist bending, which causes a problem in that the displacement of the diaphragm unit is reduced and/or unwanted heat is generated.
Japanese Unexamined Patent Application Publication No. 2001-260348 discloses an inkjet recording head using a diaphragm unit having a unimorph structure. In this case, the thicknesses of a diaphragm and a piezoelectric element are appropriately set such that a neutral plane is positioned in the diaphragm so as to prevent a portion of the piezoelectric element from moving so as to act against bending.
On the other hand, with a diaphragm unit in which a diaphragm is bonded to a piezoelectric element having a bimorph structure, it is impossible to position the neutral plane in the diaphragm regardless of the thicknesses of the diaphragm and the piezoelectric element, because the piezoelectric element itself bends. Therefore, a portion of the piezoelectric element moves so as to act against bending, thereby decreasing the amount of displacement and the pump efficiency and increasing unwanted heat generation.